OCR · GCSE · Physics

    Course version: J249 · Version from Sept 2018

    Describe ways to increase efficiency

    Full specification wording

    Power and efficiency: describe ways to increase efficiency (Higher tier only)

    Efficiency rises when a smaller share of the input energy is dissipated, usually as heat to the surroundings.

    Checked against the OCR J249 specification (opens in a new tab) on .

    • Point 6 of 8
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Efficiency rises when a smaller share of the input energy is dissipated, usually as heat to the surroundings. Ways to achieve this include lubricating moving parts so friction transfers less energy to thermal stores, reducing air resistance by streamlining shapes, and insulating or lagging a device so less heat escapes. Improving the design of a machine, such as using bearings or tighter seals, also cuts wasted transfers. Each measure works by reducing the wasted energy for the same useful output, so the ratio of useful output to total input increases. Explain the mechanism, not just the change: state which wasteful transfer is reduced and why efficiency then rises.

    What earns the marks

    • States that lubricating moving parts reduces friction and so reduces energy dissipated as heat.
    • Explains that streamlining reduces air resistance and therefore wasted energy transfer.
    • Describes insulation or lagging as reducing heat loss to the surroundings.
    • Links each measure to a smaller wasted transfer for the same useful output, raising the useful-to-total ratio.
    • Identifies the wasteful transfer reduced and explains why efficiency increases as a result.

    Common mistakes and how to put them right

    • Claims a measure increases the total energy input.

      Put it right: Explain instead that it reduces the wasted output.

    • Lists a change such as lubrication without saying which wasteful transfer it reduces.

      Put it right: Name the friction and heat loss.

    • Assumes efficiency can reach 100% in real devices.

      Put it right: Recognise that some dissipation always remains.

    • Confuses increasing power output with increasing efficiency.

      Put it right: Link the change to the useful-to-total ratio.

    Exam tips

    • For each measure, name the wasteful transfer it reduces and state the effect on the efficiency ratio.
    • Use precise terms such as friction, air resistance and thermal dissipation rather than 'less waste'.
    • Give a concrete device example, such as lubricating engine parts or lagging a boiler.

    Try the lesson

    The start of the Power and efficiency: describe ways to increase efficiency (Higher tier only) lesson, made from checked teaching notes for this specification point.

    Have a go first

    A cyclist notices that energy is wasted as heat and sound when the bicycle's chain rubs against the gears. Which action would reduce this waste and improve efficiency?

    Listen

    Efficiency rises when a smaller share of the input energy is dissipated, usually as heat to the surroundings. Ways to achieve this include lubricating moving parts so friction transfers less energy to thermal stores, reducing air resistance by streamlining shapes, and insulating or lagging a device so less heat escapes. Improving the design of a machine, such as using bearings or tighter seals, also cuts wasted transfers. Each measure works by reducing the wasted energy for the same useful output, so the ratio of useful output to total input increases. Explain the mechanism, not just the change: state which wasteful transfer is reduced and why efficiency then rises. A strong answer earns marks for these points. States that lubricating moving parts reduces friction and so reduces energy dissipated as heat. Explains that streamlining reduces air resistance and therefore wasted energy transfer. Describes insulation or lagging as reducing heat loss to the surroundings. Links each measure to a smaller wasted transfer for the same useful output, raising the useful-to-total ratio. Exam tip: For each measure, name the wasteful transfer it reduces and state the effect on the efficiency ratio.

    See it

    A top-down flowchart. A single node labelled 'Measure to reduce wasted energy' branches into four measures: 'Lubricate moving parts', 'Streamline shapes', 'Insulate or lag the device', and 'Improve design: bearings, tighter seals'. Each measure leads to its effect: lubrication gives 'Less friction, less energy dissipated as heat'; streamlining gives 'Less air resistance, less wasted energy transfer'; insulation gives 'Less heat loss to the surroundings'; improved design 'Cuts wasted transfers'. All four effects feed into one node: 'Smaller wasted transfer for the same useful output'. That node leads to the final node: 'Useful-to-total ratio increases, so efficiency rises'.
    It shows the measures that reduce wasted energy and how each one raises the useful-to-total ratio, increasing efficiency.

    Key words

    Say what each word means before you turn the card.

    Reducing friction

    Try answering aloud or in your head before turning the card.

    Reducing air resistance

    Try answering aloud or in your head before turning the card.

    Reducing heat loss by insulation

    Try answering aloud or in your head before turning the card.

    Wasted energy and the efficiency ratio

    Try answering aloud or in your head before turning the card.

    See every definition
    Reducing friction
    Lubricating moving parts so friction transfers less energy to thermal stores, which reduces the energy dissipated as heat.
    Reducing air resistance
    Streamlining shapes so air resistance is reduced and therefore less energy is transferred wastefully.
    Reducing heat loss by insulation
    Insulating or lagging a device so less heat escapes to the surroundings.
    Wasted energy and the efficiency ratio
    Each measure reduces the wasted energy for the same useful output, so the ratio of useful output to total input increases.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A single node labelled 'Measure to reduce wasted energy' branches into four measures: 'Lubricate moving parts', 'Streamline shapes', 'Insulate or lag the device', and 'Improve design: bearings, tighter seals'. Each measure leads to its effect: lubrication gives 'Less friction, less energy dissipated as heat'; streamlining gives 'Less air resistance, less wasted energy transfer'; insulation gives 'Less heat loss to the surroundings'; improved design 'Cuts wasted transfers'. All four effects feed into one node: 'Smaller wasted transfer for the same useful output'. That node leads to the final node: 'Useful-to-total ratio increases, so efficiency rises'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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